Week 7 State of charge estimation

Aim:

    To simulate the downloaded model and explain the results

Objective:

1. Simulate the 3 test cases from harness dashboard and write a detailed report on the results
2. What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation ?

Link of the model:

https://in.mathworks.com/matlabcentral/fileexchange/72865-design-and-test-lithium-ion-battery-management-algorithms

Model:

The model file is opened as shown below

Blocks:

1) Dashboard

• Test sequence varient
• Manual varient 
• BMS_Testsequence state request(Subsystem) to provide state request commands.
• Lamps to indicate Fault state,High temperature,Over voltage,Over current,charging ,voltage sensor operation,inverter operation,low temperature,Under voltage.If any fualt in these red indication will intimate us and all in good conditions indicated by green indication.
• Rotating switch for manual Varient changes while running simulation.

In scope results the SOC of module,Module current,Individual cell voltages,Individual cell temperature,Battery state,Cell balancing states are seen as outputs.

2) BMS_Closed loop

• In this main block consist of two subsystems,BMS ECU and PLANT units.
• The plant output from plant unit given as input to BMS ECU as BMS_input feedback loop.The state request is given to BMS ECU unit.
• The BMS Output commands given as input to plant unit as 'From_BMS'.Alsp the state request is given to  Plant unit.

The final output from the closed loop model are

• BMS_inputs
• BMS_output
• BMS-info

In BMS ECU unit,the current power limit calculations are done used in State machine block,Soc Estimations,Cell balancing logics as shown below, 

• In Plant unit,the battery pack unit,Pre charge unit,charger/load units are created.
• In battery packs there are two types of configurations as 1 module(6 cells) and 16 modules(96 cells).
• Here we are chossing 1 module for lesser simulation running time.

Objective 1

   To run all the three cases frim harness dashboard of closed loop BMS control

Test 1:

Here from the above test sequence editor figure it was set as,

• Driving for first 3000 secs.
• After 3000sec starts balancing operations occure for next 1000 Secs.
• Then charging and balancing occurs for next 5000 secs.
• After that sequences post charge balancing operation occurs till end of the simulation period.

Result:

After the simulation the results are shown below

The green lamp or light indicates that in the overall simulation period there are no faults occureing during the sequence the red mark show indicatres the process of balancing of the cells.

The graph is shown below

Understanding the result

Cell Voltages and Pack Current:

• For first 3000secs all 6 cells are in drive mode where discharging occurs majorly and in standby mode for lesser duration at approximately 1000 to 2000 secs.
• The voltages varies from 3.1V(min) to 3.9V(max) in this range.
• From 3000 secs to 4000secs cell balancing occurs and in this stage the module in standby mode and voltage is constant at nearly 3.7V.
• After 4000secs charging occurs,voltage raised from 3.7V to 4V. At 4V to 4.2V at constant current as nearly 30 A and after 4.2 V as constant voltage,current reduces from 30A to 2.6A 
• at end of the charging and cell balancing occurs upto 9000Secs.
• After 9000 secs post charge cell balancing starts. All cell voltages are balancing to a voltage with respect to cell 1 to 4.1 V as shown below,in this stage the pack is in stand by mode.

Cell temperatures:

• Initially for 3000Secs the temperature range of cells increases gradually from ambient temperature 288K to 302K.
• The temperature of cell 1 is lesser and cell 6 has higher temberature for all stages.
• After 3000secs upto 4000secs the cell temperatures are reduced slightly as no current flows through cell in standby mode.
• After 4000Secs temperature starts increases exponentially upto constant current region and start reduces as current flow reduces in constant voltage and cell balancing region upto 9000secs.
• Cell 1 max temperature raised upto 302K and cell 6 temperature raised upto 317K.  
• After 9000secs the temperatures still reduces and remains stable at reaching ambient temperature.

BMS State:

• For 3000 secs the battery module in drive mode(discharging).
• From 3000 secs to 4000secs the battery module in balancing standby mode as shown from result plot.
• After 4000 secs upto 9000secs the battery module in charging and balancing mode. 
• After 9000secs only post charge balancing mode operates upto end of simulation.

State Of Charge SOC:

• Initially the state of charge of battery module is in 80%. 
• Here the yellow plot represents SOC estimation done by coulomb counting technique.
• Blue plot represents SOC estimation using Unscented Kalman filter technique.
• Red plot represents SOC estimation using Extended Kalman filter technique. Among three UKF and EKF techniques estimate accurately than coulomb counting technique.
• Initially for 3000 secs SOC decreases to nearly 45% during drive mode.
• After 3000secs upto 4000 secs the SOC is constant at 45%  as cell balancing occurs in standby mode.
• From 4000 secs to 9000secs module in charging mode.(4000 to 7000=CC mode,7000 to 9000=CV mode).
• After 9000 secs the SOC becomes constant approximately 94%  as seen from above plots.

Cell Balancing:

Initially upto 3000 secs balancing was not performing.

• After 3000 secs cell balancing starts with on cell 6 and cell 2 as they rised to 1 as shown above since its voltage is higher than all other cell voltages.
• After this stage charging and balancing,post charge balancing are occurs as Cell 6 and cell 2 remains in 1 state till end of simulation. 
• As cell 1(yellow) is in lesser voltage than other cells it remains in 0 state through out the simulation.  
• At the end of the simulation all cell voltages try to balance equally as we seen earlier in single cell voltages plot.

Test 2

The test sequence is selected 

This test case consist of drive mode for 10000secs and after that charging occurs till end of simulation.

The model simulation is done and the result is shown below

Results:

The graph:

Here in these plots only driving mode of battery module is plotted.

Cell Voltages and Pack Current-

• The cell voltages varies from maximum 3.9V and reduces to 3.05V as cutoff voltage upto 9000 secs.Entire capacity drained at 9000 secs.
• The pack current discharges upto maximum 70A  and slight charging occurs during the overall drive.
• Some stand by mode conditions are in between the drive modes.

Cell temperature:

• Here cell 1 having lesser temperature variations from 288K to approximately 302.5K.
• Cell 6 is having higher temperature variation from 288K to approximately 322.5K.
• The cell temperatures are slightly reducing in between the drive modes as these durations are in stand by modes. 

Battery module state:

• Entire sequence is performing in drive mode as shown from the above plot.
• Small duration of standby modes are not plotted in this plot as simulation upto 10000secs command is given as drive mode.
• But in actual practical situation standby modes are not present present as battery current drains by other small auxilaries of EV's.

SOC of module:

Here the SOC reduces from 80% to 0% (as current drained for longer duration) in entire drive mode as shown in above plot.

At stand by modes inbetween them the SOC remains constant as shown.

Balancing:

Here balancing of cells are not performed as shown as balance states are in 0 for entire drive mode.

Test 3:

We are selecting the case three from th eblock parameter as we have done earlier.

The test case and the operation done by the test is shown below

The test consists of the following

The mode is simulated and the result is shared below

Result:

The dashboard image shown

The graph:

Cell voltages and pack current:

• Initially cell 2 , voltages rises from 3.9V(approx) to maximum 4.2V and stabilised to 4.15V.
• Cell 1 reises from 3.85V to 4.1V and stabilised to 4.08V.
• The battery pack current raising from 0A to 30A and starts to  reduce to 2.5 A gradually at end of full charging upto 2000 secs.
• At 2000secs the pack is fully charged to 100%. 
• After full charge current flow will be cutoff  as shown in result plot.

Cell Temprature:

• The cell temperatures continue to increase heat upto full charge and after that the temperatures get started to reduces to ambient temperature.
• As shown from result plot the temperatures start decreases after full charge(at 2000 secs).

BMS:

The BMS is constant as shown above:

SOC:

• The pack SOC gets fully charged at 2000secs and remains at full charge for rest of the sequence as shown above.
• The actual state of charge is around 93% shown as full charge shown by UKF and EKF techniques.

Cell Balancing:

Here from 0 sec the cell 6 and 2 starts to balance with respect to cell 1 and all cells are try to balancing at end of simulation.

Conclusion:

Hence from above three test cases the performance of BMS on various factors are discussed and analysed.

Objective 2:

What is coulomb counting? Refer to the above model and explain how BMS implements coulomb counting for SOC estimation?

Coulomb counting:

• SOC(State Of Charge) is defined as the rate of available capacity to its maximum capacity when a battery is fully charged and describes the remaining percentage of the battery
• capacity.
• To find this SOC % we are using coulomb counting method.
• The coulomb counting method measures the discharging current of the battery and integrates the discharging current over time in order to estimate SOC.
• Which can be estimated from discharrging current(I(t)),previously estimated SOC values(SOC(t-1)).The relation can be given as,

 

Flow Chart:

Modified Coulomd counting:

• In this technidue discharge current is corrected with a function and corrected current is found out for accurate SOC estimation.
• The function can be given as,

Where K0,K1,K2 are constant values obtained from the practice experimental data.

The SOC relation can be given as,

SOC estimation in BMS model:

To see the model of coulomb counting technique in BMS model click the subsystem of SOC_Estimation and click Coulomb counting.

• Here the discharge current from battery given to gain with value as 1/3600 to convert current to Ahr.
• The value is divided by capacity due to temperature change. Then integrated with a gain value of 1 with sample time -1 as we are using discrete integrator.
• The saturation limits are from 0 to 1.
• We will get the SOC values from 0 to 1 multiplied with 100 to get in percentage of SOC